1. Field of the Invention
This invention relates generally to the evaluation of a wafer of semiconductor material, and in particular to the measurement of a property of the semiconductor material.
2. Description of Related Art
In the processing of a semiconductor wafer to form integrated circuits, charged atoms or molecules are directly introduced into the wafer in a process called ion implantation. Ion implantation normally causes damage to the lattice structure of the wafer, and to remove the damage, the wafer is normally annealed at an elevated temperature, typically 600xc2x0 C. to 1100xc2x0 C. Prior to annealing, material properties at the surface of the wafer may be measured, specifically by using the damage caused by ion implantation.
For example, U.S. Pat. No. 4,579,463 granted to Rosencwaig et al. (that is incorporated herein by reference in its entirety) describes a method for measuring a change in reflectance caused by a periodic change in temperature of a wafer""s surface (see column 1, lines 7-16). Specifically, the method uses xe2x80x9cthermal waves [that] are created by generating a periodic localized heating at a spot on the surface of a samplexe2x80x9d (column 3, lines 54-56) with xe2x80x9ca radiation probe beam . . . directed on a portion of the periodically heated area on the sample surface,xe2x80x9d and the method xe2x80x9cmeasure[es] the intensity variations of the reflected radiation probe beam resulting from the periodic heatingxe2x80x9d (column 3, lines 52-66).
As another example, U.S. Pat. No. 4,854,710 to Opsal et al. (also incorporated herein by reference in its entirety) describes a method wherein the density variations of a diffusing electron-hole plasma are monitored to yield information about features in a semiconductorxe2x80x9d (column 1, lines 61-63). Specifically, Opsal et al. state that xe2x80x9cchanges in the index of refraction, due to the variations in plasma density, can be detected by reflecting a probe beam off the surface of the sample within the area which has been excitedxe2x80x9d (column 2, lines 23-31) as described in xe2x80x9cPicosecond Ellipsometry of Transient Electron-Hole Plasmas in Germanium,xe2x80x9d by D. H. Auston et al., Physical Review Letters, Vol. 32, No. 20, May 20, 1974.
Opsal et al. further state (in column 5, lines 25-31 of U.S. Pat. No. 4,854,710): xe2x80x9cThe radiation probe will undergo changes in both intensity and phase. In the preferred embodiment, the changes in intensity, caused by changes in reflectivity of the sample, are monitored using a photodetector. It is possible to detect changes in phase through interferometric techniques or by monitoring the periodic angular deflections of the probe beam.xe2x80x9d
A brochure entitled xe2x80x9cTP-500: The next generation ion implant monitorxe2x80x9d dated April, 1996 published by Therma-Wave, Inc., 1250 Reliance Way, Fremont, Calif. 94539, describes a measurement device TP-500 that requires xe2x80x9cno post-implant processingxe2x80x9d (column 1, lines 6-7, page 2) and that xe2x80x9cmeasures lattice damagexe2x80x9d (column 2, line 32, page 2). The TP-500 includes xe2x80x9c[t] wo low-power lasers [that] provide a modulated reflectance signal that measures the subsurface damage to the silicon lattice created by implantation. As the dose increases, so does the damage and the strength of the TW signal. This non-contact technique has no harmful effect on production wafersxe2x80x9d (columns 1 and 2 on page 2). According to the brochure, TP-500 can also be used after annealing, specifically to xe2x80x9coptimize . . . system for annealing uniformity and assure good repeatabilityxe2x80x9d (see bottom of column 2, on page 4).
A method in accordance with this invention: (1) creates charge carriers in a concentration that changes in a cyclical manner (also called xe2x80x9cmodulationxe2x80x9d) only with respect to time, in a region (also called xe2x80x9cilluminated regionxe2x80x9d) of a semiconductor material, and preferably also (2) maintains the charge carriers at an average concentration that remains the same (or at least approximately the same e.g. varies less than 10%) before and during a measurement indicative of the number of charge carriers created in the illuminated region by act (1).
In one embodiment (also called xe2x80x9cscanning embodimentxe2x80x9d), one or more such measurements are compared each with the other, thereby to identify a sudden change in the measurements. In another embodiment (also called xe2x80x9cmeasurement embodimentxe2x80x9d), one or more measurements are compared with similar measurements on wafers (also called xe2x80x9creference wafersxe2x80x9d) processed under known conditions and having known properties, thereby to determine one or more process conditions or properties of a wafer under fabrication.
In one implementation, an attribute derived from measurements on a wafer is interpolated with respect to corresponding attributes of wafers having a known material property (or process condition), thereby to determine a corresponding property (or condition) of the wafer under measurement. An example of a process condition is the temperature (also called xe2x80x9cannealing temperaturexe2x80x9d) at which the wafer is annealed. Examples of material properties include surface concentration, mobility, junction depth, lifetime and defects that cause leakage current at the junction (when the junction is reversed biased).
The charge carriers (also called xe2x80x9cexcess carriersxe2x80x9d) being created and measured as described above are in excess of a number of charge carriers (also called xe2x80x9cbackground charge carriersxe2x80x9d) that are normally present in the semiconductor material (e.g. due to dopant atoms) even in the absence of illumination. Therefore, in the first act described above, a number of excess carriers are created in the above-discussed region (also called xe2x80x9cilluminated regionxe2x80x9d), e.g. by focusing thereon a laser beam or an electron beam. The concentration of excess carriers is modulated, both at the surface and in the bulk only as a function of time (e.g. by modulating the intensity of the just-described laser or electron beam that is also called xe2x80x9cgeneration beamxe2x80x9d).
The frequency of modulation of the concentration of excess carriers is deliberately selected to be sufficiently low to avoid modulation in space (i.e. avoid the creation of a wave of charge carriers). A carrier concentration that is devoid of a wave in space is created when at least a majority of the charge carriers (i.e. greater than 50%) move out of the illuminated region by diffusion. Such a temporal modulation under diffusive conditions (also called xe2x80x9cdiffusive modulationxe2x80x9d) is used to measure the reflectance caused by excess carriers, e.g. by detection of the intensity of a beam (also called a xe2x80x9cprobe beamxe2x80x9d) reflected by the illuminated region at the modulation frequency.
In the second act, an average concentration (e.g. root mean square average) of the excess carriers is determined from a measurement of the above-described reflectance over the time period of a modulation cycle. The average concentration is maintained the same (or approximately the same) prior to and during the measurement of reflectance. Specifically, the creation of new charge carriers (also called xe2x80x9cmeasurement-relatedxe2x80x9d carriers) in addition to the background charge carriers and the excess carriers is minimized or avoided during the reflectance measurement, thereby to maintain the total carrier concentration at or about the just-described average prior to the measurements.
An apparatus (also called xe2x80x9cprofilerxe2x80x9d) that implements the above-described method includes, in one embodiment, a source that produces a probe beam formed of photons of energy lower than the bandgap energy (the energy necessary to generate conduction electrons) of the semiconductor material. Use of such a probe beam source eliminates the measurement-related carriers and the resulting errors that are otherwise created by a prior art apparatus, e.g. in measuring the reflectance with a probe beam that has photons of energy greater than the bandgap energy of silicon (such as the Hexe2x80x94Ne laser probe beam described at column 15, line 56 of U.S. Pat. No. 4,854,710).
In addition to the above-described probe beam source, the profiler also includes a photosensitive element (such as a xe2x80x9cphotodiodexe2x80x9d) that is located in the path of a portion of the probe beam reflected by the illuminated region. The photosensitive element generates an electrical signal (e.g. a voltage level) that indicates the intensity of the probe beam portion reflected by the illuminated region. The intensity in turn indicates reflectance caused by the excess charge carriers (e.g. created by incidence of a generation beam).
So, in one embodiment, the intensity measurement is used by the profiler as a measure of the concentration of excess charge carriers in the illuminated region. In this embodiment, the profiler also includes a computer that is coupled to the photosensitive element to receive the electrical signal, and that is programmed to determine the value of a material property in the illuminated region from one or more such measurements.
In another embodiment, the profiler creates measurement-related carriers by use of a probe beam having photons at or slightly above the bandgap energy of the semiconductor material. Even in the presence of such measurement-related carriers, the profiler maintains the overall accuracy of a measurement of a material property (as described herein) within a predetermined limit (e.g. 10% error) by limiting the number of such measurement-related carriers to a small percentage (e.g. up to 10%) of the excess carriers.
In the just-described embodiment, a diffusive modulation of charge carriers is created by use of a generation beam that has a wavelength and power chosen so that the rate of carriers generated by this beam is sufficiently larger, e.g. one order (preferably two orders) of magnitude larger than the rate of carriers generated by the probe beam so as to make the latter negligible.
In one implementation, in addition to the above-described probe beam source, the profiler includes a source that produces a generation beam (formed of photons) having an intensity that is modulated at a sufficiently low frequency to avoid creation of a wave of charge carriers. The powers of the two beams are maintained by the profiler to be at least approximately the same. Other implementations have two beams that each have a power different from the other, and yet maintain the measurement-related carriers (created by the probe beam) at a negligible percentage (e.g. the probe beam has photons of energy higher than the bandgap energy but has a power that is half or one-fourth the power of the generation beam, assuming that the power of the reflected portion of the probe beam is detected with sufficient accuracy as described above).
Measurement of intensity of a reflected portion of the probe beam while (1) using diffusive modulation and (2) generating negligible (preferably none) percentage of measurement-related carriers is a critical aspect of one embodiment of the invention. One or more such measurements provide a measure of a process condition or a property of the semiconductor material in the illuminated region. Such measurements are performed in one embodiment after annealing to activate the dopants, thereby to obtain a measure that is more indicative of the electrical behavior of the devices being fabricated than a property that is measured prior to annealing (as described in U.S. Pat. No. 4,854,710).
The above-described intensity measurements (from which reflectance measurements are derived), and one or more properties (also called xe2x80x9cmaterial propertiesxe2x80x9d) are preferably (but not necessarily) monitored during fabrication, to control a process step (e.g. to control annealing temperature of a wafer that has been ion implanted) used in fabricating a wafer. As the material properties are measured directly on the wafer undergoing fabrication (also called xe2x80x9cpatterned waferxe2x80x9d or xe2x80x9cannealed waferxe2x80x9d depending on the stage of fabrication), a measurement as described herein increases yield, as compared to an off-line measurement of a test wafer""s properties.
During operation, the profiler (described above) performs a number of reflectance measurements, each measurement being for a different value of a parameter used either (1) in the generation of the excess carriers or (2) in the measurement of the concentration of excess carriers. In one embodiment, the profiler fits two or more such measurements to one or more straight lines or to a curved line, and compares an attribute of the fitted line (e.g. compares a first order coefficient, also called xe2x80x9cslope,xe2x80x9d of the fitted line), with corresponding attributes of corresponding lines generated from such measurements on wafers having known properties, thereby to determine a material property corresponding to the fitted line.
Moreover, a process condition (e.g. the temperature at which a patterned wafer has been annealed) can also be determined from such comparison of reflectance measurements, if the process condition affects the semiconductor material. Depending on the implementation, the comparison can be performed either manually or by a computer.
In a first implementation, the parameter varied between the measurements is the average concentration of charge carriers that is controlled by, e.g., changing the power or the diameter of the generation beam used to generate the charge carriers. Thereafter, a material property, such as mobility (or junction depth) is determined from intensity measurements by comparison of the above-described attribute with the corresponding attributes of wafers having known mobilities (or junction depths).
In one example, the wafer is undoped and mobility is determined by computing the slope of a plot of the intensity measurement against the power of the generation beam. In another example, the wafer has doped regions, and mobility is determined by comparing a slope obtained from the measurements (in the same manner as the just-described slope for the undoped wafer) with slopes of wafers having known mobilities.
In a second implementation, the varied parameter is the distance between the two beams that are used in performing an intensity measurement. In two variants of this implementation, the location of either (1) the generation beam, or (2) the probe beam, is changed relative to the wafer. The material property determined from such intensity measurements is lifetime.
In a third implementation, the parameter that is varied is the relative size of the two beams used in the intensity measurement e.g. the diameter of the probe beam is changed (while keeping the power of the probe beam the same). In this implementation as well, the material property determined from the intensity measurements is lifetime.
In another embodiment (also called xe2x80x9cpolarized embodimentxe2x80x9d), a laser beam that is linearly polarized is used as a probe beam (also called xe2x80x9cpolarized probe beamxe2x80x9d). The polarized probe beam need not have photons of energy below the bandgap energy of the semiconductor material, i.e. a beam of photons at or slightly above the bandgap energy (as described above) can be used if polarized.
On reflection from the illuminated region, a plane of polarization of the probe beam rotates through two different angles, depending on the following two reflection coefficients: one coefficient in a plane (also called xe2x80x9csurface planexe2x80x9d) of the surface of the illuminated region, and another coefficient in a plane (also called xe2x80x9cnormal planexe2x80x9d) perpendicular to the surface plane.
After reflection, a portion of the probe beam that has been reflected is interfered with another portion that was not reflected. Next, two measurements are made, specifically of the sum and difference signals generated by the interference. Thereafter, a difference (hereinafter xe2x80x9cin-phase differencexe2x80x9d) between the two measurements that is in phase with the modulation of charge carriers is determined using a phase detector. The in-phase difference signal provides a measure of the concentration of excess carriers in the illuminated region.
Thereafter, one or more of the material properties discussed above are determined by use of in-phase difference signal (instead of using the intensity measurement described above). Use of a polarized probe beam (as described herein) provides an increase in sensitivity of the measurement of material properties by about two orders of magnitude over the use of a non-polarized beam, because of the increased sensitivity of a phase detector used in the polarized embodiment (as compared to an amplitude detector that is otherwise used) to measure the power of the reflected portion of the probe beam.